time: _2006 is a literal _, followed by 2006

Otherwise _2006 is treated as _2 and then an error.

Fixes #11334

Change-Id: I40a385b45e279e9f4538bf419baab72781cdb215
Reviewed-on: https://go-review.googlesource.com/16311Reviewed-by: Rob Pike <r@golang.org>

src/time/format.go

@@ -162,8 +162,12 @@ func nextStdChunk(layout string) (prefix string, std int, suffix string) {
 			}
 			return layout[0:i], stdDay, layout[i+1:]
 
-		case '_': // _2
+		case '_': // _2, _2006
 			if len(layout) >= i+2 && layout[i+1] == '2' {
+				//_2006 is really a literal _, followed by stdLongYear
+				if len(layout) >= i+5 && layout[i+1:i+5] == "2006" {
+					return layout[0 : i+1], stdLongYear, layout[i+5:]
+				}
 				return layout[0:i], stdUnderDay, layout[i+2:]
 			}
 

src/time/format_test.go

@@ -529,3 +529,22 @@ func TestFormatSecondsInTimeZone(t *testing.T) {
 		}
 	}
 }
+
+// Issue 11334.
+func TestUnderscoreTwoThousand(t *testing.T) {
+	format := "15:04_20060102"
+	input := "14:38_20150618"
+	time, err := Parse(format, input)
+	if err != nil {
+		t.Error(err)
+	}
+	if y, m, d := time.Date(); y != 2015 || m != 6 || d != 18 {
+		t.Errorf("Incorrect y/m/d, got %d/%d/%d", y, m, d)
+	}
+	if h := time.Hour(); h != 14 {
+		t.Errorf("Incorrect hour, got %d", h)
+	}
+	if m := time.Minute(); m != 38 {
+		t.Errorf("Incorrect minute, got %d", m)
+	}
+}